Contactless detection of a relative angular position is a widely used application of sensors in industry. In particular, when detecting the angle of rotation magnetically in a contact-free manner, an indicator element, which produces a magnetic field, is connected to a rotor and the magnetic field, which changes as a function of the angular position of the indicator element, of a field probe is detected, the field probe being mounted on a stator, so as to be stationary relative to the indicator element. Alternatively, it is known to mount the indicator element, which produces the magnetic field, so as to be stationary and to position the field probe so as to be movable relative the indicator element. Only the relative movement between the indicator element and the field probe is significant for the principles according to the invention which are described in the following. It is thus substantially irrelevant which of the two parts is to be considered the stator and which the rotor. Generally, however, the stationary installation of the field probe is to be preferred due to the electrical signal processing required.
It is thus known to use a permanent magnet with an annular cross-section as the magnetic field-generating indicator element, as shown in Prior Art FIG. 9. In its known embodiment, the permanent magnet 900 is formed by two diametrically magnetized portions 902, 904. The arrows 906 indicate the magnetic field lines which, as is generally known, are directed from the magnetic north pole to the magnetic south pole. As shown in Prior Art FIG. 9, the inner region 908 of the permanent magnet 900 is, however, permeated by a comparatively inhomogeneous magnetic field, with this diametrical polarization of the magnetization.
This has the drawback that, on the one hand, a single field probe, which is arranged in this region to detect a relative rotational movement between the permanent magnet and the field probe, must be accommodated as exactly as possible at the center of said inner region 908 in order to obtain a reproducible linear signal. The arrangement is therefore particularly sensitive to positional tolerances such as axis misalignment or axis tilting.
On the other hand, the known arrangement cannot be used to produce a redundant rotation angle sensor principle in which more than one field probe is arranged in the interior 908 of the indicator element, because there is not sufficient space available with identical field conditions.
Now, a discussion of known magnetic characterizations is provided. In general, what is known as the MR effect (magnetoresistive effect) occurs in all conductive materials. The electrical resistance thus increases when a magnetic field is applied, because the charge carriers are deflected from their straight movement and this leads to an increase in the length of the path. In principle, the magnetoresistive effect only achieves useful changes in resistance at very high magnetic field strengths in materials which are good conductors, such as copper. Bismuth has the greatest resistance lift of any metal. In specific semiconductor materials, which are generally referred to as magnetoresistors, changes in resistance of more than 100% can be achieved.
What is known as the anisotropic magnetoresistive effect (AMR effect) occurs in magnetic materials. The resistivity thereof is a few percent greater parallel to the magnetization than perpendicular thereto. The magnetization can be simply rotated in thin sheets made of said material in such a way that sensors can be produced. In 1998, what is known as the giant magnetoresistive effect (GMR effect) was discovered and it led to a development which resulted in the discovery or rediscovery of further MR effects, which are conventionally combined under the collective term XMR in the literature.
The GMR effect occurs in the layer systems with at least two ferromagnetic layers and a metal intermediate layer. If the magnetization in said layers is not parallel, the resistance is greater than if the magnetization were parallel. This can involve a difference of up to 50%, thus resulting in the name “giant”.
What is known as the tunnel magnetoresistive effect (TMR effect) occurs in layer systems with at least two ferromagnetic layers and a thin insulating layer. The tunnel resistance between the two layers is dependent on the angle of the two magnetization directions to one another, exactly as with the GMR effect.
The colossal magnetoresistive effect (CMR effect) is a volume effect and occurs predominantly in perovskite materials. At temperatures in the vicinity of their transition temperature from metallic to semiconductor behavior, changes in resistance of more than 200% have been observed. Until now, however, this effect is only known to occur in materials having a transition temperature of less than 100 Kelvin.
The giant magnetic inductance effect (GMI effect) predominantly occurs in wires which have a surface layer made of a magnetic material. Said layer must have magnetization annularly about the wire. Said magnetization is also rotated in the wire direction by magnetic fields in the longitudinal direction of the wire. The inductance of the wire thus changes, in particular at high frequencies, since the skin effect is influenced by the magnetic layer. This effect can also be observed in the case of magnetic double layers, although to a substantially lower extent.
In contrast to the known Hall elements, which measure the magnetic flow and are thus susceptible to interference in terms of geometric factors, dimensions, the influence of temperature variations, and material variations, magnetoresistive sensors are operated with a saturated soft magnetic layer and a magnet, which is saturated during magnetization, and only the direction of the field, but not the flux density, is measured. A particular minimum flux density is thus always provided. Therefore, the use of magnetoresistive sensors offers the advantage of greatly increased robustness and resistance to interference, which is important, in particular for use in motor vehicles.